I sympathize with you b/c my daughter can be like this too. I am lucky in that most of the time, she at least gets a long morning nap of 1-3 hours, but after that it's always 30 minute naps the rest of the day. And you could set a clock to her, 30 minutes exactly LOL. But I feel your pain, b/c when she doesn't take a long nap in the morning and I have to deal with those all day, you just can't get anything done. I wish I had a secret tip for you to make your DS nap longer, but I can't figure out why my DD sometimes naps long in the morning, and sometimes doesn't.
My DD used to nap in the swing, that's the only place we could get her to sleep in the day, now we've moved her to a raised seat that sits in the crib (she has acid reflux, so we keep her elevated 30-60 minutes after feedings). And she naps fine there after about 2 days of adjusting to that. Some babies take up to a week or two to adjust to a change in their napping spot, so have you given him enough time to adjust (without changing it again?)--I assume you have, but that's the only thing I can think of!
Good luck. Perhaps as he gets older his naps will change naturally too (hopefully longer).